N = int(raw_input())
g = set([])
for i in range(N):
    x, y = map(int, raw_input().split())
    if x >= y:
        print -1
        break
    else:
        g.add(y - x)
        if len(g) > 1:
            print -1
            break
    
    if i == N - 1:
        print g.pop()